Documentation: iio: Document ad7606 driver

The Analog Devices Inc. AD7606 (and similar chips) are complex ADCs that
will benefit from a detailed driver documentation.

This documents the current features supported by the driver.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-3-654faf1ae08c@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>